New coatings are continuously being developed in order to increase the lifetime of the coated cutting tools, increase machining speeds and to improve the quality of the machined surface. Longer tool life at high cutting speed increases productivity while reducing costs of maintenance and personnel. Use of cutting tools that maintain hardness at high working temperatures with minimum lubricant or for dry machining both reduces costs and is environmentally friendly.
It is known (US 2007/0184306) to coat cutting tools with a hard film composed of M1-a-bAlaSib(BxCyN1-x-y) where M denotes Ti and Cr. The lower and upper limits of the atomic ratio of Al (a) in this film are 0.05 and 0.5 respectively. This hard coating film has a crystal structure free of hexagonal crystals and/or amorphous phase since, according to the inventors, the presence either the hexagonal phase or the amorphous phase decreases the hardness of the hard coating film.
A cutting tool coated with at least one layer formed out of an amorphous compound phase including Si, and compounded by Ti, Al, Si with relatively rich Si and with at least one type of element selected from the C, N, O, B, and a crystalline compound phase compounded by Ti, Al, Si with relatively poor Si with at least one type of element selected from C, N, O, B is disclosed in JP2002337002.
Two phase nano-composite coatings of at least 1 μm having nano-crystalline (nc) grains of either nc-TiN, nc-(Al1-xTix)N, or nc-(Al1-xCrx)N that are “glued” together by an amorphous (a) matrix of a-Si3N4 are known. (Veprek et al. Thin Solid Films 476 (2005) 1-29). The Si atoms of the a-Si3N4 are covalently bonded to the nitrogen showing Si 2p binding energy of 101.7±0.1. The amorphous Si3N4 matrix has a shear and de-cohesion strength greater than that of bulk SiNx.
One advantage of this coating is the increased hardness having a Vickers hardness HV of over 40 GPa. The generic concept for the design of superhard and thermally stable nano-composites is based on a thermodynamically driven spinodal phase segregation that results in the formation of a stable nanostructure by self-organization. In order to achieve this during deposition, a sufficiently high-nitrogen activity (partial pressure≧0.02 mbar) at relatively high deposition temperature (500-600° C.) is needed. The nitrogen provides a high thermodynamic driving force and the temperature ensures diffusion-rate controlled phase segregation to proceed sufficiently fast during deposition to obtain the phase segregation. Thermodynamic calculations of the Gibbs free energy of a mixed system of stoichiometric TiN and Si3N4 show that under the deposition temperature and nitrogen pressure described above, the phase segregation is of a spinodal nature (R. F. Zhang, S. Veprek/Materials Science and Engineering A 424 (2006) 128-137 and S. Veprek et al./Surface & Coatings Technology 200 3884 (2006) 3876-3885). Thus, the nano-crystalline phase should not have any Si content when deposited at these conditions.
Another advantage in nano-composite coatings of nc-(Al1-xTix)N/a-Si3N4 is that the thin a-Si3N4 matrix stabilizes the Al-rich (Al1-xTix)N metastable solid solution against the decomposition into cubic c-TiN and hexagonal h-AlN and concomitant softening of the coating. It is known that (Al1-xTix)N begins to decompose at temperatures of about 700° C. in (Al1-xTix)N coatings that do not contain Si. Nano-composite layers of nc-(Al1-xTix)N/a-Si3N4 are stable up to 1200° C.
Another important advantage of nano-composites is their high oxidation resistance up to temperatures of more than 800° C. This is related to the dense and strong a-Si3N4 matrix that hinders the diffusion of oxygen along the grain boundaries. However, different modes of measuring oxidation resistance are used so that it is not easy to compare the published “starting oxidation temperatures” and the temperatures at which the oxidation is critical. For example, in one method (US2007/0184306) the sample was heated in dry air at a rate of 4° C./min and the weight increase due to oxidation was plotted. The temperature at which the sample began to increase in weight was regarded as the oxidation starting temperature. Another method (Veprek et at Surface & Coatings Technology 202 (2008) 2063-5073) is to compare the thickness of the oxide formed on a nano-composite coating and on another coating, for example TiAlN, after one hour in air at a given temperature, for example 900° C. Another method, similar to the first, takes the temperature at which the oxide can first be seen at high magnification (of about 1000×) as the oxidation starting temperature.
A known multilayer coating is of a nano-composite AlCrSiN top layer that is 0.75-1.5 μm thick on a more ductile and softer underlayer of TiAlN that is 3-5 μm thick. (S. Veprek et al, Surface & Coatings Technology 202 (2008) 2063-2073). The method of deposition uses vacuum arc coating technology with planar cathodes. Another method of deposition uses vacuum arc coating technology for an nc-(TiAl)N/a-Si3N4 nano-composite. In this method rotating cathodes placed either in the center of the coating chamber or in its door. Other known methods of deposition include unbalanced magnetron sputtering.
CN101407905 discloses a coated cemented carbide cutting tool the coating comprising a composite mixed crystal structural layer containing a layer of a nano crystalline/amorphous composition made up of TiAlMSiN deposited on a titanium based binding layer. M is one or more of metal elements of Ta, Nb, Zr, Cr, Hf, and W. The thickness of the TiAlMSiN layer is at least 0.5 μm.
A known disadvantage of nano-composite coatings is that impurities for example oxygen, even at levels of a few hundred ppm, lead to a very strong degradation of the maximum achievable hardness.
Another disadvantage is a limitation of nano-composite layer thickness. These layers are known to have high compressive stresses and therefore have a tendency to flake if they are thicker than about 3 μm.
Another disadvantage of nano-composite coatings with enhanced hardness is relatively low toughness (Plasma Process. Polym. 2007, 4 219-228 Zhang et. al.). Toughness is the ability of a material to absorb energy during deformation up to fracture. In order to obtain high hardness in nano-composite coatings, usually plastic deformation is designed to be prohibited, and grain boundary and sliding are prevented, thus causing a loss of ductility. Ductility is related to toughness, which is very important for hard coatings to avoid catastrophic failure.